Duty cycle control for motor/LED in Embedded C - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this PWM duty cycle calculation?

Given the following code snippet that calculates the PWM duty cycle for a motor, what is the value of duty_cycle after execution?

Embedded C

int max_pwm = 255;
int speed_percent = 60;
int duty_cycle = (speed_percent * max_pwm) / 100;

A 255

B 153

C 60

D 100

💡 Hint

Remember that duty cycle is a fraction of max PWM value based on percentage.

The duty cycle is calculated by multiplying the speed percentage (60) by the max PWM value (255) and dividing by 100, which results in 153.

❓ Predict Output

intermediate

What is the LED brightness value after this PWM update?

Consider this code controlling LED brightness with PWM. What is the value of led_brightness after running this code?

Embedded C

int max_pwm = 1023;
int brightness_percent = 25;
int led_brightness = (brightness_percent * max_pwm) / 100;

A 256

B 255

C 1023

D 25

💡 Hint

Calculate the percentage of max PWM value.

25% of 1023 is 255.75, which truncates to 255 in integer arithmetic, but the actual integer division results in 255.75 truncated to 255. However, the calculation (25 * 1023) / 100 = 255.75 truncated to 255, so option A is correct.

❓ Predict Output

advanced

What error does this PWM duty cycle code produce?

What error will occur when compiling or running this code snippet?

Embedded C

int max_pwm = 255;
float speed_percent = 75.5;
int duty_cycle = (speed_percent * max_pwm) / 100;

A TypeError: cannot assign float to int without cast

B No error, duty_cycle = 192

C SyntaxError: invalid expression

D Warning: implicit conversion from float to int, duty_cycle = 192

💡 Hint

Check how float to int conversion works in C.

In C, assigning a float expression to an int variable causes implicit conversion with truncation, usually with a warning but no error. The value is 192 after truncation.

🧠 Conceptual

advanced

Which statement best describes PWM duty cycle effect on motor speed?

Select the statement that correctly explains how PWM duty cycle affects motor speed.

A Higher duty cycle means the motor receives power for a longer time, increasing speed.

B PWM frequency controls speed, duty cycle controls torque.

C Duty cycle does not affect motor speed, only voltage does.

D Lower duty cycle increases motor speed by reducing power interruptions.

💡 Hint

Think about how power delivery time affects motor rotation.

PWM duty cycle controls how long the motor receives power in each cycle. A higher duty cycle means more power time, increasing speed.

❓ Predict Output

expert

What is the final value of duty_cycle after this code runs?

Analyze this embedded C code that adjusts PWM duty cycle dynamically. What is the final value of duty_cycle?

Embedded C

int max_pwm = 255;
int duty_cycle = 0;
for (int i = 0; i < 5; i++) {
    duty_cycle += (i * max_pwm) / 10;
}
if (duty_cycle > max_pwm) {
    duty_cycle = max_pwm;
}

A 0

B 127

C 254

D 255

💡 Hint

Calculate the sum inside the loop and check the cap condition.

The loop adds (i * 255) / 10 for i=0 to 4: 0 + 25 + 51 + 76 + 102 = 254. Since 254 <= 255, the cap condition is not met, so duty_cycle = 254 (A).

Practice

1. What does the duty cycle control in a motor or LED?

easy

A. The percentage of time the device is ON during each cycle

B. The voltage supplied to the device

C. The frequency of the power supply

D. The temperature of the device

Solution

1. Step 1: Understand duty cycle meaning

   The duty cycle is the fraction of one period in which a signal or system is active.

2. Step 2: Relate to motor/LED control

   For motors or LEDs, it controls how long the device is ON in each cycle, affecting speed or brightness.

3. Final Answer:

   The percentage of time the device is ON during each cycle -> Option A

4. Quick Check:

   Duty cycle = ON time percentage [OK]

Hint: Duty cycle means ON time percentage per cycle [OK]

Common Mistakes:

• Confusing duty cycle with voltage
• Thinking it controls frequency
• Assuming it controls temperature

2. Which of the following is the correct way to set a 50% duty cycle using a timer compare register in embedded C?

easy

A. OCR0 = 0; // Full OFF

B. OCR0 = 255; // Full ON

C. OCR0 = 128; // Assuming 8-bit timer with max 255

D. OCR0 = 64; // 25% duty cycle

Solution

1. Step 1: Understand timer resolution

   An 8-bit timer counts from 0 to 255, so max value is 255.

2. Step 2: Calculate 50% duty cycle value

   50% of 255 is approximately 128, so setting OCR0 to 128 sets 50% duty cycle.

3. Final Answer:

   OCR0 = 128; // Assuming 8-bit timer with max 255 -> Option C

4. Quick Check:

   50% of 255 = 128 [OK]

Hint: Half of max timer value sets 50% duty cycle [OK]

Common Mistakes:

• Using max value for 50% duty cycle
• Using zero for 50% duty cycle
• Confusing 25% with 50%

3. Given the code snippet below, what will be the duty cycle percentage set for the LED?

unsigned char duty = 64;
unsigned char max = 255;
float dutyCycle = (duty / (float)max) * 100;

medium

A. 25%

B. 50%

C. 75%

D. 64%

Solution

1. Step 1: Calculate duty/max ratio

   Divide duty (64) by max (255): 64 / 255 ≈ 0.25098.

2. Step 2: Convert ratio to percentage

   Multiply by 100: 0.25098 * 100 ≈ 25.1%.

3. Final Answer:

   25% -> Option A

4. Quick Check:

   (64/255)*100 ≈ 25% [OK]

Hint: Divide duty by max, multiply by 100 for percentage [OK]

Common Mistakes:

• Using integer division without casting
• Confusing duty value with percentage
• Rounding incorrectly

4. Identify the error in the following code that attempts to set a 75% duty cycle on an 8-bit timer:

unsigned char duty = 75;
unsigned char max = 255;
OCR0 = duty / max * 255;

medium

A. OCR0 register cannot be assigned directly

B. Integer division causes duty/max to be zero

C. max value should be 100, not 255

D. Duty cycle calculation is correct

Solution

1. Step 1: Analyze division operation

   Both duty and max are unsigned char, so duty/max does integer division: 75/255 = 0 (integer).

2. Step 2: Effect on OCR0 assignment

   Multiplying 0 by 255 results in 0, so OCR0 is set to 0, not 75% duty cycle.

3. Final Answer:

   Integer division causes duty/max to be zero -> Option B

4. Quick Check:

   Integer division truncates to zero [OK]

Hint: Cast to float before division to avoid zero result [OK]

Common Mistakes:

• Ignoring integer division rules
• Assuming OCR0 cannot be assigned
• Using wrong max value

5. You want to smoothly increase the brightness of an LED from 0% to 100% duty cycle using PWM on an 8-bit timer. Which approach is best to implement this in embedded C?

hard

A. Use a fixed OCR0 value of 128 for half brightness

B. Set OCR0 to 255 once and leave it

C. Toggle the LED pin ON and OFF manually without PWM

D. Use a loop to increment OCR0 from 0 to 255 with a delay between steps

Solution

1. Step 1: Understand smooth brightness control

   Smooth brightness change requires gradually changing the duty cycle from 0 to max.

2. Step 2: Implement gradual PWM change

   Increment OCR0 from 0 to 255 in a loop with delays to create smooth brightness ramp.

3. Final Answer:

   Use a loop to increment OCR0 from 0 to 255 with a delay between steps -> Option D

4. Quick Check:

   Gradual OCR0 change = smooth brightness [OK]

Hint: Increment OCR0 gradually with delay for smooth fade [OK]

Common Mistakes:

• Setting OCR0 once for full brightness only
• Using fixed mid value for all brightness
• Not using PWM for brightness control